A mobile land vehicle capable of traversing a wide variety of difficult terrain while remaining difficult to disable is an attractive concept for many applications including search and rescue operations, reconnaissance missions, sentry duty, and planetary exploration. A spherical vehicle in which the outer surface is the driving surface and using internal mass shifting as a method of propulsion, has great potential for such applications since it is always right side up, and the shell protects the components, all of which are located inside. However, such vehicles are limited in their acceleration and in their ability to ascend inclines, and climb over obstacles.
Pendulum-type ball robots comprising a main axle diametrically connected to the shell of a ball and supporting a drive mechanism adapted to drive a weighted pendulum for rotation around the main axle are known. With the center of mass below the center of the ball, torque can be applied to the main shaft or axle to tilt the pendulum forward, thereby shifting the center of mass in a forward direction. On a level surface, this action moves the center of mass in front of the sphere's contact point with the ground, causing the ball to roll forward as a result of gravitational forces. By continuously maintaining the pendulum at an angle, thrust may be provided, thereby enabling the sphere to roll continuously.
Tilting the pendulum in a backwards direction will cause the sphere to move backwards, while tilting the pendulum to one side while the sphere is moving forward or backward will cause the sphere to travel in an arc to the left or right. Since the drive mechanism interfaces with the shell at two places, the internal and external surface dimensions of the shell are not critical, permitting the shell to be constructed in several ways and provide compliance for shock absorption.
Propulsion by shifting the center-of-gravity of a spherical vehicle to cause it to roll places limitations on the vehicle's ability to ascend inclines and overcome obstacles, limited by optimization of the placement of the mass, and how much the center-of-gravity can be shifted internally. U.S. Patent Publication No. 2008/0097644 for “Ball Robot” published on 24 Apr. 2008, describes a ball robot which lowers the center of mass of the vehicle by hanging the driving unit(s) on at least one pendulum as close to the inner surface of the shell of the robot as possible, and states that from a stopped condition, obstacles can be traversed if their height is less than about 25% of the radius of the ball, and that the ability to traverse such an obstacle is proportional to the ratio between the distance from the center of the sphere to the center of mass, to the sphere radius.
Such limitations may have prevented spherical vehicles from achieving the importance expected from their un-invertible characteristic in that they may become stuck in shallow valleys, or stopped by small obstacles.